#!/usr/bin/pthon3
grade = input("please input grade:")
try
    grade = float(grade)
except ValueError:
    print ("not numbers")
    exit()
if grade.isdigit():
    if 90 <= float(grade) <= 100:
        print ("a")
    elif 80 <= float(grade) <= 89:
        print ("b")
    elif 70 <= float(grade) <= 79:
        print ("c")
    else:
        print ("d")
else:
    print ("please input number")
